cffi: use ffi.from_buffer() in ZstdCompressor.compress() (#26)
cffi: use ffi.from_buffer() in ZstdCompressor.compress() (#26)

Without this, we may not accept all objects implementing the
buffer protocol.
indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard